The manufacture of semiconductor devices often requires the formation of electrical conductors on semiconductor wafers. For example, electrically conductive leads on the wafer are often formed by electroplating (depositing) an electrically conductive material such as copper on the wafer and into patterned trenches.
Electroplating involves making electrical contact with the wafer surface upon which the electrically conductive layer is to be deposited (hereinafter the "wafer plating surface"). Current is then passed through a plating solution (i.e. a solution containing ions of the element being deposited, for example a solution containing Cu.sup.++) between an anode and the wafer plating surface (the wafer plating surface being the cathode). This causes an electrochemical reaction on the wafer plating surface which results in the deposition of the electrically conductive layer.
Generally, electroplating systems use soluble or insoluble anodes. Insoluble anodes tend to evolve oxygen bubbles which adhere to the wafer plating surface. These oxygen bubbles disrupt the flow of ions and electrical current to the wafer plating surface creating nonuniformity in the deposited electrically conductive layer. For this reason, soluble anodes are frequently used.
Soluble anodes are not without disadvantages. One disadvantage is that soluble anodes, by definition, dissolve. As a soluble anode dissolves, it releases particulates into the plating solution. These particulates can contaminate the wafer plating surface, reducing the reliability and yield of the semiconductor devices formed on the wafer.
One conventional technique of reducing particulate contamination is to contain the soluble anode in a porous anode bag. However, while preventing large size particulates and chunks from being released into the plating solution, conventional anode bags fail to prevent smaller sized particulates from entering the plating solution and contaminating the wafer plating surface.
Another conventional technique of reducing particulate contamination is to place a filter between the anode and the article to be electroplated as set forth in Reed, U.S. Pat. No. 4,828,654 (hereinafter Reed). Referring to FIG. 2 of Reed, filters 60 are positioned between anode arrays 20 and a printed circuit board 50 (PCB 50). Filters 60 allows only ionic material of a relatively small size, for example one micron, to pass from anode arrays 20 to PCB 50. While allowing relatively small size particulates to pass through, filters 60 trap larger sized particulates avoiding contamination of PCB 50 from these larger sized particulates. Over time, however, filters 60 become clogged by these larger sized particulates.
To reduce clogging of filters 60, Reed provides a counterflow of plating solution through filters 60 in a direction from PCB 50 towards anode arrays 20. This counterflow tends to wash some of the larger sized particulates from filters 60. However, even with the counterflow, eventually filters 60 become clogged. To allow servicing of filters 60, retaining strips 66 and support strips 68 allow filters 60 to be removed and cleaned when filters 60 eventually become clogged.
Although providing a convenient means of cleaning filters 60, removal of filters 60 necessarily releases the larger sized particulates from within the vicinity of anode arrays 20 into the entire system and, in particular, into the vicinity where PCBs 50 are electroplated. Even after filters 60 are cleaned and replaced, this contamination of the system can cause contamination of a subsequently electroplated PCE 50 reducing the reliability and yield of the printed circuit boards. Further, even with filters 60, particulates accumulate on receptacle 14 in the vicinity of anode arrays 20 and the system must periodically be shut down and drained of plating solution to clean these particulates from receptacle 14.
In addition to creating particulates, a soluble anode changes shape as it dissolves, resulting in variations in the electric field between the soluble anode and the wafer. Of importance, the thickness of the electrically conductive layer deposited on the wafer plating surface depends upon the electric field. Thus, variations in the shape of the soluble anode result in variations in the thickness of the deposited electrically conductive layer across the wafer plating surface. However, it is desirable that the electrically conductive layer be deposited uniformly (have a uniform thickness) across the wafer plating surface to minimize variations in characteristics of devices formed on the wafer.
Another disadvantage of soluble anodes is passivation. As is well known to those skilled in the art, the mechanism by which anode passivation occurs depends upon a variety of factors including the process conditions, plating solution and anode material. Generally, anode passivation inhibits dissolution of the anode while simultaneously preventing electrical current from being passed through the anode and should be avoided.